I think the long tail isn't an issue. We'll probably have dual stack equipment for the next few decades. What matters is getting the large websites on IPv6 so that people browsing on mobile (usually NAT64) or developing nations (usual CGNAT) can get a non-sucky web experience.

Heck, if all the sites on that page where green, mobile Internet would suddenly be a lot better since the carrier NAT boxes would have to deal with ~90% less traffic.

The big issue here are AWS/azure dragging their heels (and I assume some CDNs, too)